Estimation of deformation temperature, flow stress, and strain rate from the veins of an internal shear zone: Insights from Pelling-Munsiari thrust, Sikkim Himalaya

 

Estimating deformation conditions from shear zone rocks is critical in understanding its complex deformation history. However, often the deformation conditions from mylonite provide information on the finite deformed state conditions. On the contrary, if there are veins preserved, they may record incremental strain stages during progressive deformation. Thus, we used veins as incremental strain markers to evaluate the spatial and temporal variation in deformation conditions along the transport direction of a major shear zone. We estimated vein attributes at the microscopic scale, deformation temperature, flow stress, and strain rate from the Pelling-Munsiari thrust in the Sikkim Himalaya. It is a regionally folded thrust that acts as the roof thrust of a complex Lesser Himalayan duplex. The PT zone is exposed as ~938 m and ~188 m thick quartz-mica mylonite zone at the hinterland-most (Mangan) and the frontal exposures (Suntaley) in eastern Sikkim, respectively. The PT zone is subdivided into three domains where the protomylonite domain is surrounded by mylonite domains on both sides.

We recognize three different vein-sets based on the angular relationship to the mylonitic foliation. At both the locations of the PT zone, the low-angle (0-30°) is the dominant vein-set followed by moderate-angle (30-60°) and high-angle (60-90°). Based on the cross-cutting relationship, we find high-angle vein set is the youngest. The low-angle vein-sets are dominant in both these locations. We observed multiple crack-and-sealed events in Mangan, indicating repeated failure and mineral precipitation. In contrast, we do not observe any such texture in the veins that are preserved in the frontal exposure of the PT zone. At both the PT zones, there are higher distribution of veins near the footwall. In the hinterland, veins record coarser grain sizes in the protomylonite domain than in the mylonite domain. However, we observed a different trend in the frontal exposure, where veins from the mylonite domain record coarser grain sizes. In both locations, quartz grains dominantly exhibit the subgrain rotation recrystallization mechanism. We semi-quantitatively estimate a first-order deformation temperature using the recalibrated quartz recrystallization thermometer (Law, 2014). In the hinterland, the low-angle vein-set records the highest deformation temperature. In contrast, high-angle veins record higher deformation temperature in the foreland. Following Stipp et al. (2003) and Twiss (1977), we estimate flow stress from recrystallized quartz grain-size piezometer. The high-angle (~24.71MPa) vein-set records the highest flow stress in the hinterland. In comparison, moderate-angle (~29.55MPa) veins record the highest flow stress in the foreland exposure. Following Hirth et al. (2001), we estimated similar strain rates (~10^-15 sec^-1) from both locations. The three sets of veins record different deformation conditions in both locations suggesting different incremental strain stages. Interestingly, the high-angle veins record the fastest strain rate (~6*10^-15 sec^-1) in the hinterland most exposure, whereas, in the frontal part the moderate-angle veins record the fastest strain rate (~9*10^-15 sec^-1).